To me they are both same and that is why i am wondering why we have dictionary data structure when we can do everything with arrays/list? What is so fancy in dictionaries?
+4
A:
Arrays provide random access of a sequential set of data. Dictionaries (or associative arrays) provide a map from a set of keys to a set of values.
I believe you are comparing apples and oranges - they serve two completely different purposes and are both useful data structures.
Most of the time a dictionary-like type is built as a hash table - this type is very useful as it provides very fast lookups on average (depending on the quality of the hashing algorithm).
Andrew Hare
2010-04-22 22:43:57
its 1:1 mapping?
itsaboutcode
2010-04-22 22:44:52
@itsaboutcode yes, although the value accessed by key can be an array/list of more values.
CrazyJugglerDrummer
2010-04-22 22:46:24
A dictionary is very similar to an array. Whereas an array maps the index to the value, a dictionary maps the key to the value.
Tilo Mitra
2010-04-22 22:46:30
A:
To build on what Andrew said, in some languages such as PHP and Javascript, the array can also function as a dictionary (known as associative arrays). It also comes down to loose v strict typing in the language.
Tilo Mitra
2010-04-22 22:45:16
+1
A:
Arraylists just store a set of objects (that can be accessed randomly). Dictionaries store pairs of objects. This makes array/lists more suitable when you have a group of objects in a set (prime numbers, colors, students, etc.). Dictionaries are better suited for showing relationships between a pair of objects.
Why do we need dictionaries? lets say you have some data you need to convert from one form to another, like roman numeral characters to their values. Without dictionaries, you'd have to hack this association together with two arrays, where you first find the position the key is in the first list and access that position in the second. This is terribly error prone and inefficient, and dictionaries provide a more direct approach.
CrazyJugglerDrummer
2010-04-22 22:45:40
if you define fancy as "simpler, more reliable, easier to understand and work with", yes :D
CrazyJugglerDrummer
2010-04-22 22:52:53
The example which you have given can be implemented by by n*2 array? infect to me dictionary is n*2 array. My interpretation is right?
itsaboutcode
2010-04-22 22:54:26
You are correct in the first part; An N*2 or 2-dimensional array is another way of storing key-value pairs. On the outside, a dictionary and a n*2 array do the same things. A dictionary just goes about the process differently internally.
CrazyJugglerDrummer
2010-04-22 23:23:40